Field of the Invention
The present invention relates to a substrate table and a lithographic apparatus.
Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a known embodiment of a lithographic apparatus, a substrate is loaded on a support surface of a substrate table by a robot which holds the substrate at the bottom side of the substrate. To make loading of the substrate on a substantially horizontal support surface possible, three loading pins, also called e-pins, are provided in the substrate table. The loading pins are movable between an extended position, wherein the upper ends of the loading pins extend above the substrate table, and a retracted position, wherein the upper ends of the loading pins are retracted in the substrate table.
During the loading of the substrate on the substrate table, the robot loads the substrate on the three loading pins in the extended position. Since the substrate will be received on the loading pins extending above the support surface, the robot can be withdrawn leaving the substrate on the loading pins.
Then, the loading pins can be moved to the retracted position to place the substrate on the support surface. The support surface is defined by upper ends of the multiple burls provided on, and extending from, a main body of the substrate table.
The shape of the substrate during the loading sequence is defined by, among other things, the gravity sag of the substrate. There is limited freedom to manipulate the final substrate shape while the substrate touches the substrate table.
As a result, different parts of the substrate will usually touch different ones of the burls of the support surface at different moments during the loading.
In the time period between the substrate first touching one or more burls and the substrate being completely supported by the burls, the shape of the substrate changes from an initial shape, caused by the substrate being supported by only three loading pins in gravity, to a final shape when supported by the multiple burls. Typically, during this time period, the outer edge of the substrate moves outwardly and slides over the burls at an outer part of the support surface. This sliding of the substrate over the burls may cause wear to the upper ends of the burls. This sliding may also cause particles to be removed from the bottom of the substrate. These particles then form contamination that may interfere with the imaging referred to above.
Also, lifting the substrate by the loading pins from the support surface will cause a sliding movement of the substrate over the burls, similarly giving rise to wear of the burls and to contamination. The wear of the burls typically occurs at the outer region of the support surface.
Wear of the burls may lead to the support surface becoming non-planar. As a consequence, the lithographic apparatus may not be able to focus correctly during the projection of the patterned beam on a target portion of the substrate.
Further, it is remarked that when loading of the substrate on the support surface, defined by the upper ends of the burls, does not result in sliding of the substrate over the burls, for example caused by a large friction between the substrate and the burls, there may be internal stresses/forces in the substrate when it is held by the support. These internal stresses are undesirable since these internal stresses may have an influence on the position and/or shape of the substrate during the projection a patterned beam on the substrate. Since this position and/or shape may be different in different projections on the same substrate, the internal stresses may adversely affect the overlay performance of the lithographic apparatus.